Field of the Invention
The present invention relates to an optical element, a projection optical system, an exposure apparatus, and a device manufacturing method.
Background
In exposure apparatuses for use in a photolithography process, EUV exposure apparatuses in which extreme ultraviolet (EUV) light is used as exposure light have been proposed as disclosed, for example, in United States Patent Application, Publication No. 2005/157384. An optical element such as a multilayer-film reflection mirror having a multilayer film capable of reflecting at least part of incident light is used in a projection optical system of a EUV exposure apparatus.
As such a multilayer film, for example, a periodic multilayer film is known. It is known that the periodic multilayer film has a high reflectance with respect to EUV light. The periodic multilayer film has a configuration in which a first layer (for example, a molybdenum layer) and a second layer (for example, a silicon layer) are formed to be periodically laminated. On the other hand, when the NA (numerical aperture) of a projection optical system is larger in order to obtain high resolution, EUV light rays are incident over a wider incident angle range. Therefore, an optical system is required to reflect incident EUV light in a wide incident angle range. However, for example, it is difficult for the multilayer film having a periodic structure to obtain a uniform reflectance in a wide angle range when light having a wavelength close to the wavelength (for example, 13.5 nm) of EUV light is incident in a wide incident angle range.
On the other hand, a wideband film is proposed in order to deal with a wavelength range in which light is incident in a wide incident angle range. As such a wideband film, for example, a non-periodic film structure having layers with a different film thickness is known. In a periodic structure, the parameters of the film structure are two, that is, the thickness of one periodic structure and the thickness ratio of a first layer to a second layer; on the other hand, in the case of a non-periodic structure film, all film thicknesses of several ten layers are parameters.
Since it is required to adjust reflectance properties with high accuracy in a EUV optical system, when forming a multilayer film, the reflectance properties of an actually formed multilayer film are evaluated, and then the film structure of the multilayer film is modified and adjusted such that the reflectance properties approach target properties. In this regard, when forming a multilayer film having a periodic structure, the film structure parameters are limited to the first layer and the second layer (i.e. two layers), and therefore it is easy to modify and adjust the reflectance properties.